The present invention relates to methods and apparatus for joining wires and, more particularly, in superconducting wires comprising a bundle of filaments of tin (Sn) and niobium (Nb) contained within a binder metal to the method of joining two wires with a superconducting joint comprising the steps of, exposing the filaments on the ends of the wires to be joined; intertwining the exposed filaments; sealing the exposed filaments in a copper-enriched chamber; and, heating the chamber to a temperature and for a time sufficient for the Sn and Nb to diffuse together sufficiently and form Nb.sub.3 Sn at their interface.
Technology related to superconducting wires, and the like, is becoming an increasingly important area of technical investigation. Because of the property of extremely low resistance, energy losses through superconducting conductors are modest. Typical known superconducting conductors are composite structures formed by complicated extruding techniques as typified by the McDonald U.S. Pat. Nos. (4,262,412 and 4,414,428). As stated by the abstract of the McDonald '412 patent, the intent is to produce a "composite rod or wire of increased strength and fineness wherein the composite is formed by reducing a lamina of two metals which have been rolled to form a cylindrical billet wherein one of the metals is in expanded form. The composite produced can be encased in copper and fabricated to produce a superconductor. Alloys contemplated for producing superconductors are Nb.sub.3 Sn, Nb.sub.3 Ga, Nb.sub.3 Ge, Nb.sub.3 Si, NbTi, V.sub.3 Ga, V.sub.3 Si, V.sub.3 Sn, V.sub.3 Al, and V.sub.3 Ge laminated on bronze, Al, Cu, Ta, or combinations thereof."
As depicted in simplified form in FIG. 1, according to one method of the above-referenced McDonald patents is to form a "jellyroll" billet of the component metals and then extrude the billet to form a thin wire 10 which, as shown in the cross-section of FIG. 2, is a composite of copper (Cu) 12 encasing a plurality of filaments generally indicated as 14. Suitable methods for producing fine superconducting wires, therefore, exist in the prior art. The same cannot be said for the joining of such wires. To use Nb.sub.3 Sn wire in Nuclear Magnetic Resonance (NMR) and other persistent mode requirement magnets, and the like, superconducting joints must be made between separate lengths of wire to close the magnetic circuit. In that regard, the prior art still employs techniques and apparatus normally used for the joining of common copper conductors, and the like. For example, as depicted in FIG. 3, the ends 16 of the wires 10 are sometimes joined by a joining metal 18 using welding or brazing techniques. An alternative prior art technique is shown in FIG. 4 wherein a metal sleeve 20 is placed over the wire ends 16 and crimped or "swaged" to compress the metal of the sleeve 20 into gripping contact with the wires 10 and, thereby, affect a mechanical joint.
While information on joint technology is, in general, not widely disseminated within the developing superconductor art, it is the understanding of the inventors herein that such techniques as shown in FIGS. 3 and 4 are used and drive the users to employ conductors with large filaments to reduce the tediousness of the joining task. For Nb.sub.3 Sn, the most popular alloy at present for low temperature superconductors, this is difficult because the large filaments become brittle in the process and, therefore, there is introduced severe limitations in wire design. This is particularly true with respect to the location of the copper stabilizer in the billet and the final drawn wire.
It is, therefore, an object of the present invention to provide a method and apparatus for use in the joining of superconductors which will permit a non-tedious joining of superconductors employing fine filaments.
It is another object of the present invention to provide a method and apparatus for use therewith which will allow the joining of superconductors in a manner which does not impose severe limitations on conductor design.
It is still another object of the present invention to provide a method and apparatus for use therewith which will allow the joining of superconductors in a manner which will not cause the filaments to become brittle as a result of the making of the joint.
Other objects and benefits of the present invention will be apparent from the description of the invention which follows hereinafter when taken in conjunction with the drawing figures which accompany it.